The main objective of this research is to establish the fact that prezygotic selection is one of the important selection mechanisms to maintain various genetic polymorphisms in nature. Special emphasis will be made for human populations, particularly for the ABO blood group system. This project requires long term efforts in time and is a long range goal. During the period of the present proposal, efforts will be made for the following specific points: 1) With the genetic and demographic informations already in my hands, together with some supplementary informations to be collected, the age-specific death-birth rates or reproductive values associated with each of the ABO types will be described, and based on this, with some other informations such as segregation frequencies, the overall fitness of each ABO type could be constructed. Special emphasis will be placed on the role of prezygotic selection in the total fitness. 2) Parental inbreeding data could be used to examine the average effect of prezygotic selection operating in a certain quantitative traits in man. With the existing data collected from Odate, Japan, together with some supplementary informations to be collected, the effect of parental inbreeding upon the height, and body weight, of their children will be examined. 3) As an experimental organism, Drosophila melanogaster will be studied for the Mr system in parallel to the human ABO study. The Mr system element exhibits several peculiar genetic phenomena, particularly distorted segregation. The frequency of this element in natural population is unbelievably high (50 percent or more), and this system seems to be really interesting and useful system in working with the components of fitness, and their interactions in the overall fitness.